Melt-texturing is a process of controlling peritectic solidification to obtain a bulk, crystalline material with a high degree of lattice orientation. In the field of high-temperature superconductors, melt-texturing is used to eliminate high angle grain boundaries in the final crystalline structure. Grain boundaries substantially eliminate the amount of current which can be carried without electrical resistance losses. Superconductors have numerous uses including, but not limited to, forming frictionless bearings or flywheels.
Conventional processes for melt-texturing superconductors begin with a composition of matter termed a “123-phase,” a “123 YBCO” or a “Y-123” because the composition generally has the formula, YBa2Cu3O7-x. Superconductors based on the YBa2Cu3O7-x system, where x #0.6, have been known since they were invented by IBM in 1986. These “high temperature superconductors” are superconducting at temperatures well above absolute zero, for example, at 77K or higher. U.S. Pat. No. 5,061,682, issued to Aksay et al., discloses a process for making conductive and superconductive ceramics including Y2BaCuO5, YBa2Cu3O7, and YBa2Cu4O8, and is incorporated by reference herein.
Conventional melt-texturing processes generally heat the 123 YBCO material above its peritectic temperature (approximately 1010° C. in air for the 123 YBCO system) to decompose the 123 YBCO into its peritectic mixture which contains a second material termed a “211-phase” or “Y-211,” because the material generally has the formula Y2BaCuO5. The Y-123 also decomposes into a liquid rich in the Y (yttrium), Ba oxides, and Cu oxides. The mixture is subsequently supercooled slowly below the peritectic temperature of the 123 YBCO material. During this cooling period, the reverse reaction occurs wherein the Y-211 (Y2BaCuO5) reacts with the yttrium, the Ba oxides, and the Cu oxides in the liquid to reform 123 YBCO, which crystallizes at the supercooling temperature. Ideally, the 123 YBCO grains would nucleate uniformly such that their orientations align perfectly. However, in the supercooling period, there are often nucleations of grains with random orientations and spontaneous, secondary nucleations occur. These secondary nucleations are referred to as parasitic grains because they consume material available for the growth of desirable grains, create high angle grain boundaries upon their intersection with each other, and reduce the superconductive efficiency of the superconducting crystals.
Numerous processes are known and used to reduce these undesirable, secondary nucleations, however, such processes are incapable of halting these secondary nucleations once they begin to form. For example, U.S. Pat. No. 5,395,820, issued to Murakami et al., discloses a conventional procedure for a standard cooling cycle of 123-phase material. The process is a variation of a conventional Melt-Powdering-Melt-Growth process. The process includes the steps of combining the 123-phase with BaCO3 and CuO to prepare a mixed powder, heating the mixed powder, melting the mixed powder to form a molten material, rapidly solidifying and cooling the molten material to form a solidified material, pulverizing the solidified material to form a fine powder, mixing 0.2 to 2.0% by weight of a platinum powder such as PtBa4Cu2Oy with the pulverized fine powder, forming a body with the resultant mixture, heating the formed body to bring it to a partially-molten state, then lastly, cooling the partially-molten formed body to finely disperse a 211-phase and the platinum compound in a crystal of an oxide superconductor comprising a rare earth metal combined with Ba2Cu3Oy, where y is a number sufficient to provide oxide superconductivity. A superconductor of 123-phase type is thus produced, and the 211-phase and platinum compound are finely dispersed in a crystal of the 123-phase. Murakami et al., however, does not address any means of destroying secondary nucleations once they form. Furthermore, the process never increases the temperature of the system above the peritectic temperature of the 123-phase once crystallization of the 123-phase begins.
U.S. Pat. No. 6,046,139, issued to Blohowiak et al., also discloses a method of making single 123 YBCO crystal superconductors. In the method, 1-25 wt % of Y2BaCuO5 (211 YBCO material), 0.05-1.0 wt % Pt, and a balance of YBa2Cu3O7-x (123 YBCO material) are combined. Pt is believed to limit the growth of the non-superconducting 211-phase crystals. The resulting powder is pressed into a compact in a disk or other configuration. A seed crystal SmBa2Cu3O7-x, where x=1.6, is in contact with and is placed substantially parallel to the compact's top surface. The compact is heated to a sintering temperature between 1010° C. and 1050° C. and held at that temperature for a time sufficient to fuse the seed crystal to the compact surface. The temperature is lowered at a rate of approximately 0.1-1.0° C. per hour. 123 YBCO crystal growth nucleates from the seed crystal as the materials cool. After nucleation, the compound is cooled at a rate of about 1-10° C. per hour to a temperature of approximately 950° C. 123 YBCO crystal growth radiates from the nucleation site until the entire compact consists essentially of single crystal, single grain 123 YBCO body. The process, however, does not increase the temperature over the peritectic temperature once crystallization begins, and therefore, there is no means of destroying secondary nucleations once they form.
Other conventional melt-texturing processes attempt to limit the detrimental secondary nucleations by slowly increasing the temperature during the growth phase. However, such processes are not very effective in growing substantial superconducting crystalline structures because the temperature is never raised over the peritectic temperature after the initial heating of the material, and therefore, the processes are incapable of sufficiently destroying secondary nucleations once they form. Moreover, this slow heating detrimentally slows growth of the crystalline structure. For example, U.S. Pat. No. 6,171,390, issued to Satoh et al., discloses a method for preparing a large oxide crystalline material wherein a Y-123 oxide superconductive crystalline precursor is added with seed crystals, supercooled below its peritectic temperature, and slowly heated while keeping the material in a supercooled condition to promote crystal growth. While the method gradually increases the temperature of the furnace during the growth stage to reduce secondary nucleations, this slow heating also reduces the crystal growth rate. Furthermore, the furnace temperature during the growth stage never reaches the peritectic temperature of the system, therefore, the process is incapable of destroying secondary nucleations once they form.
Other such melt-textured growth processes have been disclosed but such processes do not include raising the temperature above the peritectic temperature after the initial heating of the material during the growth phase of the superconducting crystal. “The Increase of the Critical Current Density of YBa2Cu3O7-y by a Modified Melt-Textured Growth Method” by Choi et al., Physica C 269, pp. 306-312 (1996) discloses a process for the melt-textured growth of superconducting crystals. The article discloses a quick dip in temperature to reduce the number of 123 nuclei after a mild increase in temperature. In the process, the 123 material is sequentially: 1) heated to 1007° C. for 20 minutes; 2) raised above the peritectic temperature to 1050° C. for 30 minutes; 3) rapidly cooled over 5 minutes to a temperature below the peritectic temperature; 4) soaked at the cooled temperature for 2 to 8 minutes; 5) raised over 2 minutes to a higher temperature (which is still below the peritectic temperature); 6) soaked at that temperature for [t] hours; 7) cooled to 960° C. at a rate of 1 to 8° C./hour; and 8) cooled to room temperature at a rate of 50° C./hour. The process may reduce the number of 123-phase nuclei during the initial slow-cooling process and also reduces the 211-phase particle size. However, the process does not disclose raising the temperature above the peritectic temperature after the initial heating and during the growth stage to promote crystal growth.
Other melt-texturing processes utilize parameters other than temperature to reduce growth of secondary nucleations. For example, U.S. Pat. No. 5,856,277, issued to Chen et al., discloses a method of manufacturing a textured layer of a high temperature superconductor via decreasing the partial pressure of oxygen in the atmosphere of the system to reduce the peritectic temperature of the system. The method includes providing an untextured high temperature superconductor material having a characteristic ambient pressure peritectic melting point, heating the superconductor to a temperature below the peritectic temperature, establishing a reduced oxygen atmospheric pressure causing a reduction of the peritectic melting point which causes melting from an exposed surface of the superconductor, and subsequent raising of the partial pressure of the oxygen in the atmosphere (while raising temperature) to cause solidification of the molten superconductor in a textured surface layer. However, once the oxygen partial pressure is increased to begin solidification of the superconductor, the process does not utilize a variation in oxygen concentration to selectively destroy detrimental, secondary nucleations that have formed in the process.
Additionally, the above mentioned processes do not disclose a means of growing numerous crystals in a single run. Typically, the above processes require expensive specialized ovens in which only one area within the oven is optimized for rapid 123-phase crystal growth with minimal secondary nucleations. Furthermore, the above-mentioned processes do not give rise to a resulting 123-phase crystalline structure having differing superconductivity properties within resulting growth bands of the crystalline structure.
Therefore, it would be desirable to provide a method for enhancing the melt-textured growth of a superconducting crystalline structure to produce a superconducting crystalline structure substantially free of secondary nucleations and having superior characteristics.